Methods of forming magnesium-based alloys having a bimodal microstructure and magnesium-based alloy components made therefrom

ABSTRACT

Methods of making magnesium-based alloy components are provided. A preform of a magnesium-based alloy having a plurality of zirconium-rich domains distributed in a magnesium-alloy matrix is subjected to a temperature of ≥ about 360° C. and a deformation process that facilitates selective dynamic recrystallization to create a bimodal microstructure in the magnesium-based alloy component having a plurality of un-recrystallized regions distributed in a matrix comprising dynamically recrystallized grains. The magnesium-based alloy includes zinc (Zn) at ≥ about 2 to ≤ about 4 wt. % of the magnesium-based alloy, zirconium (Zr) at ≥ about 0.62 wt. % to ≤ about 1 wt. % of the magnesium-based alloy, total impurities at ≤ about 0.1 wt. % of the magnesium-based alloy, and a balance of magnesium (Mg). Hot-formed magnesium-based alloy components formed from such methods are also contemplated, including automotive components.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit and priority of Chinese Application No. 2022106071 34.4 filed May 31, 2022. The entire disclosure of the above application is incorporated herein by reference.

INTRODUCTION

This section provides background information related to the present disclosure which is not necessarily prior art.

The present disclosure relates to methods of making magnesium-based alloy components, such as automotive components, by subjecting a magnesium-based alloy to a hot plastic deformation process to facilitate dynamic recrystallization and to create a bimodal microstructure having a plurality of un-recrystallized domains distributed in a matrix comprising dynamically recrystallized grains for enhanced strength and ductility.

Lightweight metal components for vehicle (e.g., automotive) applications are often made of aluminum and/or magnesium alloys. Such lightweight metals can form load-bearing components that are strong and stiff, while having good strength and ductility (e.g., elongation). High strength and ductility are particularly important for vehicles, like automobiles. While magnesium alloys are an example of lightweight metals that can be used to form structural components in a vehicle, in practice, the use of magnesium alloys may be limited. While magnesium alloys can be treated by a variety of different formation techniques like wrought processes such as extrusion, rolling, forging, flow forming, stamping, and the like, magnesium may have relatively low strength and ductility/elongation levels as compared to aluminum.

Thus, there is an ongoing need for improved formation processes to form improved higher strength, higher ductility magnesium metal components.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

The present disclosure relates to method of making a magnesium-based alloy component having a bimodal microstructure. In one variation, a method may include casting a magnesium-based alloy by melting a magnesium-based alloy in a furnace having a temperature (T) of greater than or equal to T=650° C.+(500×((C_(Zr)−0.6))° C., where C_(Zr) represents a concentration of zirconium (Zr) at greater than or equal to about 0.62 wt. % to less than or equal to about 1 wt. % of the magnesium-based alloy, and the magnesium-based alloy further includes zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 4 wt. % of the magnesium-based alloy, total impurities at less than or equal to about 0.1 wt. % of the magnesium-based alloy, and a balance of magnesium (Mg). The method also includes solidifying the magnesium-based alloy into a preform including a plurality of zirconium-rich domains distributed in grains of a magnesium-alloy matrix. The method further includes subjecting the preform to a temperature of greater than or equal to about 360° C. and a deformation process that facilitates selective dynamic recrystallization to create a bimodal microstructure in the magnesium-based alloy component to form a plurality of un-recrystallized regions distributed in a matrix including dynamically recrystallized grains having an average size of greater than or equal to about 0.5 micrometers to less than or equal to about 10 micrometers.

In one aspect, after the subjecting the preform to the temperature of greater than or equal to about 360° C., a plurality of nanoparticles including zirconium and zinc are formed that are precursors to the plurality of un-recrystallized regions formed after the deformation process.

In one aspect, the subjecting the preform to the temperature of greater than or equal to about 360° C. and the deformation process occur concurrently.

In one aspect, the casting is conducted at a temperature (T) of greater than or equal to about 700° C. to minimize formation and settling of the plurality of the solid particles including zirconium in the molten magnesium-based alloy.

In one aspect, the deformation process is selected from the group consisting of: extruding, forging, flow forming, and combinations thereof.

In one aspect, the magnesium-based alloy includes zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 3.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy.

In one aspect, the magnesium-based alloy includes zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 2.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy.

In one aspect, the magnesium-based alloy component has the plurality of un-recrystallized regions homogeneously distributed in the matrix.

In one aspect, the plurality of un-recrystallized regions is greater than or equal to about 15% by area to less than or equal to about 40% by area of the magnesium-based alloy component and the plurality of un-recrystallized regions has an average equivalent diameter of greater than or equal to about 10 micrometers to less than or equal to about 100 micrometers.

In one aspect, at least one region of the magnesium-based alloy component has a yield strength of greater than or equal to about 170 MPa and an elongation of greater than or equal to about 15%.

In one aspect, at least one region of the magnesium-based alloy component has a yield strength of greater than or equal to about 185 MPa and has an elongation of greater than or equal to about 20%.

In one aspect, the magnesium-based alloy component is an automotive component.

The present disclosure also relates to a hot-formed solid magnesium-based alloy component. The hot-formed solid magnesium-based alloy component may include a bimodal microstructure having a plurality of un-recrystallized domains distributed in a matrix including dynamically recrystallized grains having an average size of greater than or equal to about 0.5 micrometers to less than or equal to about 10 micrometers. The magnesium-based alloy includes zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 4 wt. % of the magnesium-based alloy, zirconium (Zr) at greater than or equal to about 0.62 wt. % to less than or equal to about 1 wt. % of the magnesium-based alloy, total impurities at less than or equal to about 0.1 wt. % of the magnesium-based alloy, and a balance of magnesium (Mg).

In one aspect, the magnesium-based alloy includes zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 3.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy.

In one aspect, the magnesium-based alloy includes zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 2.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy.

In one aspect, the plurality of un-recrystallized domains is homogeneously distributed in the matrix.

In one aspect, the plurality of the plurality of un-recrystallized domains is greater than or equal to about 15% by area to less than or equal to about 40% by area of the hot-formed solid magnesium-based alloy component and the plurality of un-recrystallized domains has an average equivalent diameter of greater than or equal to about 10 micrometers to less than or equal to about 100 micrometers.

In one aspect, at least one region of the hot-formed solid magnesium-based alloy component has a yield strength of greater than or equal to about 170 MPa.

In one aspect, at least one region of the hot-formed solid magnesium-based alloy component has an elongation of greater than or equal to about 15%.

In one aspect, at least one region of the hot-formed solid magnesium-based alloy component has a yield strength of greater than or equal to about 185 MPa and has an elongation of greater than or equal to about 20%.

In one aspect, the hot-formed solid magnesium-based alloy component is an automotive component.

In one aspect, the hot-formed solid magnesium-based alloy component is a wheel.

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1 is a phase diagram for a magnesium-zinc-zirconium based alloy;

FIG. 2 shows a mechanism for forming a bimodal microstructure via dynamic recrystallization of a magnesium-zinc-zirconium based alloy according to certain aspects of the present disclosure;

FIG. 3 shows a magnesium-based alloy wheel component for a vehicle that may be formed via casting, extrusion, forging and flow forming;

FIGS. 4A-4C. FIG. 4A shows an optical microscopy (OM) scan, FIG. 4B shows a scanning electron microscopy scan (SEM), and FIG. 4C shows an electron backscatter diffraction (EBSD) scan of a bimodal microstructure formed in accordance with certain aspects of the present disclosure. Scale bar is 50 micrometers;

FIGS. 5A-5B. FIG. 5A shows a comparative magnesium-based alloy after casting, including a schematic on the left and an optical microscopy scan on the right. FIG. 5B shows an example of an inventive magnesium-based alloy prepared in accordance with certain aspects of the present disclosure after casting including a schematic on the left and an optical microscopy scan on the right. Scale bars are 100 micrometers in FIGS. 5A and 5B; and

FIGS. 6A-6B. FIG. 6A shows a comparative magnesium-based alloy after hot deforming, including a schematic on the left and an optical microscopy scan on the right. FIG. 6B shows an example of an inventive magnesium-based alloy prepared in accordance with certain aspects of the present disclosure having a bimodal microstructure after deforming including a schematic on the left and an optical microscopy scan on the right. Scale bars are 100 micrometers in FIGS. 6A and 6B.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific compositions, components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, elements, compositions, steps, integers, operations, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Although the open-ended term “comprising,” is to be understood as a non-restrictive term used to describe and claim various embodiments set forth herein, in certain aspects, the term may alternatively be understood to instead be a more limiting and restrictive term, such as “consisting of” or “consisting essentially of.” Thus, for any given embodiment reciting compositions, materials, components, elements, features, integers, operations, and/or process steps, the present disclosure also specifically includes embodiments consisting of, or consisting essentially of, such recited compositions, materials, components, elements, features, integers, operations, and/or process steps. In the case of “consisting of,” the alternative embodiment excludes any additional compositions, materials, components, elements, features, integers, operations, and/or process steps, while in the case of “consisting essentially of,” any additional compositions, materials, components, elements, features, integers, operations, and/or process steps that materially affect the basic and novel characteristics are excluded from such an embodiment, but any compositions, materials, components, elements, features, integers, operations, and/or process steps that do not materially affect the basic and novel characteristics can be included in the embodiment.

Any method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed, unless otherwise indicated.

When a component, element, or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other component, element, or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein to describe various steps, elements, components, regions, layers and/or sections, these steps, elements, components, regions, layers and/or sections should not be limited by these terms, unless otherwise indicated. These terms may be only used to distinguish one step, element, component, region, layer or section from another step, element, component, region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first step, element, component, region, layer or section discussed below could be termed a second step, element, component, region, layer or section without departing from the teachings of the example embodiments.

Spatially or temporally relative terms, such as “before,” “after,” “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially or temporally relative terms may be intended to encompass different orientations of the device or system in use or operation in addition to the orientation depicted in the figures.

Throughout this disclosure, the numerical values represent approximate measures or limits to ranges to encompass minor deviations from the given values and embodiments having about the value mentioned as well as those having exactly the value mentioned. Other than in the working examples provided at the end of the detailed description, all numerical values of parameters (e.g., of quantities or conditions) in this specification, including the appended claims, are to be understood in certain embodiments as being modified by the term “about” whether or not “about” actually appears before the numerical value, while in other embodiments, are precisely or exactly the value or parameter specified. “About” indicates that the stated numerical value allows some slight imprecision (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If the imprecision provided by “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters. For example, “about” may comprise a variation of less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in certain aspects, optionally less than or equal to 0.1%. By way of example, if a range is specified to be greater than or equal to about A to less than or equal to about B, this encompasses not only the stated range, but also a range that includes greater than or equal to exactly A to less than or equal to exactly B, as well greater than exactly A to less than exactly B in other embodiments.

As used herein, unless otherwise indicated, amounts expressed in weight and mass are used interchangeably, but should be understood to reflect a mass of a given component.

Example embodiments will now be described more fully with reference to the accompanying drawings.

Magnesium alloys include magnesium-zinc alloys, which include magnesium (Mg) and zinc (Zn), as well as zirconium (Zr). Such alloys may have moderate strength due to the strengthening effect of zinc (Zn). In traditional magnesium-zinc-zirconium alloys, such as ZK30 alloy and ZK60 alloy, the functionality of zirconium addition is for grain refinement in casting. For example, ZK30 alloy contains a nominal amount of zinc (Zn) at about 3 wt. % and zirconium (Zr) at about 0.5 to about 0.6 wt. % with a balance of magnesium and impurities. It is well-accepted that when zirconium (Zr) addition exceeds 0.5 wt. %, grain size will not be further reduced despite increased zirconium (Zr) content. Considering that zirconium (Zr) addition into magnesium alloys is costly, zirconium (Zr) content in magnesium-zinc-zirconium alloy has been controlled in the range of 0.5 to about 0.6 wt. %. Hot formed magnesium-zinc-zirconium alloy, such as ZK30 alloy has excellent formability, but its strength tends to be unsatisfactory for use in certain applications. In a conventional manufacturing process that involves casting the ingot, followed by extruding, forging, and then flow forming, a final microstructure of the formed ZK30 part may have a yield strength of only about 155 MPa and an elongation of about 13%.

In accordance with various aspects of the present disclosure, methods of forming magnesium-based alloy components having a unique bimodal microstructure are provided. The methods provided herein enable the formation of components comprising magnesium-based alloys having relatively high yield strength and relatively high elongation/ductility. The magnesium-alloys are selected to have a composition that can be processed in a manner that facilities formation of pre-solidified particles comprising zirconium in the melt during casting. In accordance with certain aspects of the present disclosure, these pre-solidified zirconium-containing particles can then serve as a nucleation site for magnesium grains during casting, where they may be partially dissolved in magnesium grains. Zirconium is supersaturated in magnesium grains and thus segregated in a core region of a grain in the as-cast microstructure. When the as-cast microstructure is further processed, for example, via heat treatment or hot forming processes, including extrusion and forging, zirconium atoms may bond with zinc atoms in the surrounding magnesium matrix to form nano-scale zirconium-containing particles. The resulting nanoscale zirconium-containing particles then impede a dynamic recrystallization process during hot plastic deformation that then facilitates formation of a bimodal microstructure in the final product. The bimodal microstructure thus formed has relatively high yield strength and relatively high elongation/ductility.

The magnesium-alloy solid components formed in accordance with certain aspects of the present disclosure are particularly suitable for use to form components of an automobile or other vehicles (e.g., motorcycles, boats, tractors, buses, motorcycles, mobile homes, campers, and tanks), but they may also be used in a variety of other industries and applications, including aerospace components, consumer goods, devices, buildings (e.g., houses, offices, sheds, warehouses), office equipment and furniture, and industrial equipment machinery, agricultural or farm equipment, or heavy machinery, by way of non-limiting example. Non-limiting examples of automotive components include hoods, pillars (e.g., A-pillars, hinge pillars, B-pillars, C-pillars, and the like), panels, including structural panels, door panels, and door components, interior floors, floor pans, roofs, exterior surfaces, underbody shields, wheels, rims, control arms and other suspension components.

The present disclosure contemplates methods of making a magnesium-based alloy component. The methods may comprise casting a magnesium-based alloy by melting the alloy and then holding the molten magnesium-based alloy in a furnace. The furnace may have a minimum temperature (T) of greater than or equal to T=650° C.+(500×((C_(Zr)−0.6))° C. C_(Zr) refers to a concentration of the zirconium in the magnesium-based alloy. For example, where a concentration of C_(Zr) is about 0.62 wt. %, a minimum temperature (T) during casting may be about 660° C. In certain variations, the minimum temperature may range from greater than or equal to about 660° C. to less than or equal to about 850° C., optionally greater than or equal to about 660° C. to less than or equal to about 750° C., and in certain aspects, greater than or equal to about 675° C. to less than or equal to about 725° C., depending on the concentration of zirconium present. In certain aspects, the temperature during casting may be selected to be greater than a phase change temperature, where solid particles comprising zirconium may be formed. In certain examples, the temperature may be greater than or equal to about 700° C. during casting. The molten magnesium-based alloy may be held at the temperature for greater than or equal to 15 minutes, for example, to permit sedimentation of oxides and other undesirable inclusions to fall to the bottom of the furnace.

However, it is desirable to avoid unnecessarily high temperatures to minimize undesirable oxidation. In one variation, a maximum temperature (T_(max)) during casting may be less than or equal to T_(max)=650° C.+(500×((C_(Zr)−0.6))° C.+80° C. By way of example, a maximum temperature during casting may be less than or equal to about 830° C., optionally less than or equal to about 800° C., optionally less than or equal to about 775° C., optionally less than or equal to about 755° C., and in certain variations, optionally less than or equal to about 740° C.

FIG. 1 shows a phase diagram 50 for a magnesium-zinc-zirconium alloy. In particular, the phase diagram 50 is for a magnesium-based alloy having 3 wt. % zinc (Zn) with varying mass/weight % levels of zirconium (Zr) shown on the x-axis 52 ranging from 0 wt. % to 1 wt. %. The y-axis 54 shows temperature in degrees (°) Celsius. A liquid phase 60 is shown at the number 1 above about 660°. When zirconium (Zr) content in the alloy is greater than 0.6 wt. % and starts solidification from a temperature above T=650° C.+(500×((C_(Zr)−0.6))° C., the alloy goes through a solidification path shown where a second zone 62 begins to form designated at 2. As can be seen at line 56, at a zirconium (Zr) concentration of 0.62 wt. % formation of the phase(s) in the second zone 62 occurs below about 660° C. As can be seen at line 58, at a zirconium (Zr) concentration of 0.65 wt. % formation of the phase(s) in the second zone 62 occurs below about 675°.

In the second zone 62, hexagonal close packed (hcp) particles comprising zirconium are formed in liquid. The solid particles are predominantly formed of zirconium, for example, having greater than or equal to about 95 wt. % up to about 100% zirconium. The solid particles comprising zirconium solidify prior to the crystallization of magnesium solid. In third zone 64, only hexagonal close packed (hcp) magnesium is formed from liquid, but no more zirconium-containing solid particles are formed. In fourth zone 66, similarly, no solid particles comprising zirconium are formed from the melt, rather hexagonal close packed (hcp) magnesium is formed from the melt. Thus, in various aspects, the magnesium-based alloy may have zirconium (Zr) at greater than or equal to about 0.62 wt. % and optionally greater than or equal to about 0.65 wt. % to facilitate formation of the pre-solidified zirconium-containing particles in the molten magnesium-alloy.

In accordance with certain aspects of the present disclosure, suitable magnesium-based alloys have a composition comprising zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 4 wt. %, optionally at greater than or equal to about 2 wt. % to less than or equal to about 3.5 wt. %, and in certain variations, and in certain variations, optionally at greater than or equal to about 2 wt. % to less than or equal to about 2.5 wt. %. As will be described below, zinc (Zn) content in the magnesium-based alloy can desirably help with pre-crystallization of particles comprising zirconium (Zr). A magnesium-based alloy may have zirconium (Zr) at greater than or equal to about 0.62 wt. % to less than or equal to about 1 wt. %, optionally greater than or equal to about 0.62 wt. % to less than or equal to about 0.8 wt. %, optionally greater than or equal to about 0.62 wt. % to less than or equal to about wt. %, optionally greater than or equal to about 0.65 wt. % to less than or equal to about 0.7 wt. %, and in certain variations, about 0.65 wt. %. As will be described in greater detail below, magnesium-alloys having such a zirconium (Zr) content can be processed to form pre-solidified particles comprising zirconium in an alloy matrix. A cumulative amount of impurities and contaminants may be present at less than or equal to about 0.3 wt. %, optionally less than or equal to about 0.1 wt. %, optionally less than or equal to about 0.05 wt. %, and in certain variations, optionally less than or equal to about 0.01 wt. % of the magnesium-based alloy. A balance of the magnesium-based alloy may be magnesium (Mg). Other alloying components may optionally be present in the magnesium alloy composition. The magnesium makes up the balance of the magnesium-based alloy, and in certain example embodiments may be greater than or equal to about 85 wt. %, optionally greater than or equal to about 90 wt. %, optionally greater than or equal to about 95 wt. %, and in certain variations, optionally greater than or equal to about 96 wt. %.

In this manner, an optimized magnesium-zinc-zirconium alloy chemistry and corresponding methods of manufacturing permit a solid component to have excellent mechanical properties after being treated via a hot forming process.

In one variation, the magnesium-based alloy comprises zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 4 wt. % of the magnesium-based alloy, zirconium (Zr) at greater than or equal to about 0.62 wt. % to less than or equal to about 1 wt. % of the magnesium-based alloy, total impurities at less than or equal to about 0.1 wt. % of the magnesium-based alloy, and a balance of magnesium (Mg). In another variation, the magnesium-based alloy consists essentially of zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 4 wt. % of the magnesium-based alloy, zirconium (Zr) at greater than or equal to about 0.62 wt. % to less than or equal to about 1 wt. % of the magnesium-based alloy, total impurities at less than or equal to about 0.1 wt. % of the magnesium-based alloy, and a balance of magnesium (Mg). The term “consists essentially of” means the magnesium alloy excludes additional compositions, materials, components, elements, and/or features that materially affect the basic and novel characteristics of the magnesium alloy, such as the magnesium alloy having the desired strength (e.g., a yield strength of greater than or equal to about 170 MPa) and/or elongation/ductility levels (e.g., an elongation of greater than or equal to about 15%), but any compositions, materials, components, elements, and/or features that do not materially affect the basic and novel characteristics of the magnesium alloy can be included in the example embodiment.

In another variation, the magnesium-based alloy comprises zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 3.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy. In a further variation, the magnesium-based alloy consists essentially of zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 3.5 wt. % of the magnesium-based alloy, zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy, total impurities at less than or equal to about 0.1 wt. % of the magnesium-based alloy, and a balance of magnesium (Mg). Any element having a minimum concentration of greater than or equal to 0 wt. % may either be absent from the magnesium-based alloy, or alternatively have a minimum concentration of 0.01%.

In yet another variation, the magnesium-based alloy comprises zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 2.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy. In a further variation, the magnesium-based alloy consists essentially of zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 2 .5 wt. % of the magnesium-based alloy, zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy, total impurities at less than or equal to about 0.1 wt. % of the magnesium-based alloy, and a balance of magnesium (Mg). Any element having a minimum concentration of greater than or equal to 0 wt. % may either be absent from the magnesium-based alloy, or alternatively have a minimum concentration of 0.01%.

In various aspects, a preform or billet of a magnesium-based alloy is formed after casting that has a plurality of magnesium grains each with a zirconium-rich core distributed therein. After the casting, the preform may be subjected to a heat treatment, for example, conducted at a temperature of greater than or equal to about 360° C., optionally in a range of greater than or equal to about 380° C. to less than or equal to about 450° C., followed by hot deformation processes. In other variations, the preform may be directly subjected to a hot deformation process. Maintaining temperatures in the range of 380-450° C. will transform the solid solution saturated zirconium to zirconium-lean magnesium matrix and nano-scale zirconium-containing particles. Nano-scale zirconium-containing particles (which may further comprise zinc) impede dynamic recrystallization process in hot forming processes and transform the original microstructure into a plurality of un-recrystallized domains distributed in a matrix comprising dynamically recrystallized magnesium-based alloy.

FIG. 2 shows a diagram of a process 100 for forming a bimodal microstructure in accordance with various aspects of the present disclosure by subjecting a preform of the magnesium-based alloy to a hot compression process, and more preferably to a deformation process, such as a hot plastic deformation process. A suitable high temperature/hot process may be conducted at a temperature of greater than or equal to about 360° C. In certain aspects, such a hot process may be a hot plastic deformation process may have a strain rate may be greater than or equal to about 0.001/s to less than or equal to about 1/s and a temperature may be greater than or equal to about 360° C. to less than or equal to about 420° C. The deformation strain may be greater than or equal to about 50% to less than or equal to about 1,000%.

In FIG. 2 , a compression or deforming process 100 begins at 102, where a preform 110 of a magnesium-based alloy component previously cast as described above is subjected to a method as described herein in accordance with certain aspects of the present disclosure. After casting of the magnesium alloy, the preform 110 has a plurality of magnesium grains each with a zirconium-rich core 114 distributed therein. As described above, magnesium grain(s) 112 are nucleated on pre-solidified particles comprising zirconium and the pre-solidified particles are dissolved into the magnesium to define the zirconium-rich core region 114 in the preform 110. Then, as heating and/or deformation 104 continues, the zirconium atoms solutioned in a magnesium core 118 will precipitate out in the form of nano-scale zinc-zirconium (ZnZr) particles 116 after heat treatment or pre-heating in a hot forming process. Alternatively, the nano-scale zinc-zirconium (ZnZr) particles 116 may precipitate out in-situ during hot forming processes, so long as temperatures requirements are satisfied, as discussed above, with temperature being greater than or equal to about 360° C.

When the preform 110 is further subjected to the deformation process a selective dynamic recrystallization process can then occur. The nano-scale zinc-zirconium (ZnZr) particles 116 impede dynamic recrystallization. Dynamic recrystallization (DRX) is the nucleation and growth of new grains that occurs during deformation, and usually at elevated temperatures. The intermediate product 120 begins to form dynamically recrystallized grains 122 around grain boundary 112. The new/recrystallized grains 122, may have different grain sizes and orientations than were previously present in the metal piece, as such the new grains may alter the mechanical properties in negative and/or positive ways. The solid nano-scale zinc-zirconium (ZnZr) particles 116 pin on low angle grain boundary (LAGB) 124 and therefore impede dynamic recrystallization from occurring via the movement of LAGB during the deformation process. As a result, core regions 118 with abundant nano-scale zinc-zirconium (ZnZr) particles 116 remain un-recrystallized. Residual coarser domains that are not dynamic recrystallized are usually regarded as undesirable for mechanical strength and ductility. However, in the context of the present disclosure, due to development of a strong texture in the un-recrystallized domains, the residual coarser domains help improve strength considerably and ductility is not sacrificed.

A final product thus formed has a plurality of dynamically recrystallized grains formed around un-recrystallized domains corresponding to core regions rich in zirconium (for example, regions that may comprise zinc-zirconium nanoparticles) that remain in an uncrystallized state. Thus, un-recrystallized regions corresponding to nano-scale zinc-zirconium (ZnZr) particles 116 are distributed within a plurality of dynamically recrystallized grains 122. A bimodal microstructure is formed that has a plurality of un-dynamically recrystallized domains formed from solid precursor particles comprising zirconium distributed in a matrix of dynamically recrystallized magnesium-alloy grains 122. Notably, FIG. 2 shows a small partial detailed section of a microstructure that would be formed on a larger scale throughout a solid component subjected to the deformation process.

In certain aspects, the dynamically recrystallized grains 122 have an average equivalent diameter of greater than or equal to about 0.5 micrometers (μm)(500 nm) to less than or equal to about 10 μm. In certain aspects, the plurality of un-recrystallized domains has an average equivalent diameter of greater than or equal to about 10 μm to less than or equal to about 100 μm. The plurality of un-recrystallized domains are formed from solid precursor particles that transform to zirconium-rich domains and later to zirconium/nano-scale zinc-zirconium (ZnZr) particles 116. The un-recrystallized domains may be greater than or equal to about 15% by area to less than or equal to about 40% by area of an overall surface area in critical stressed portion of a magnesium-based alloy component. In yet other variations, the magnesium-based alloy component has the plurality of un-recrystallized domains from core regions comprising zirconium/nano-scale zinc-zirconium (ZnZr) particles homogeneously distributed in the matrix.

In various aspects, the methods of the present disclosure may include a semi-continuous casting process in which magnesium alloy ingots and master alloy ingots may be introduced to a semi-continuous caster to form an ingot or preform. The caster may include a first holding furnace and a cast tooling downstream having a cooling system and a hydraulic or mechanical ram. In certain aspects, the casting is conducted at a temperature (T) that exceeds the phase change for forming the solid particles comprising zirconium (e.g., a temperature above the second zone 62 with reference to FIG. 1 ). In certain variations, the casting may be conducted at any of the temperatures described above, such as at a temperature of greater than or equal to about 700° C., to minimize formation and/or settling of the plurality of the solid particles comprising zirconium that are formed during the holding period in the molten magnesium-based alloy. The melt temperature in the holding furnace may be strictly controlled via heating and cooling systems to minimize formation and/or avoid settling of pre-solidified zirconium-containing particles in the molten alloy within the furnace. After the molten magnesium-based alloy flows into cast tooling, the desired zirconium-containing particle forms in situ in the melt and then acts as nucleation site for magnesium grains. The zirconium-containing particles are gradually dissolved into magnesium grains after nucleation of magnesium grains, resulting in the microstructure containing magnesium grains with zirconium-rich cores/domains for further hot processing.

The methods provided herein enable the formation of components comprising magnesium alloys by hot deformation of magnesium alloy cast billet/ingots. After forming the ingot or preform having the microstructure containing magnesium grains with zirconium-rich domains, the preform is subjected to a temperature of greater than or equal to about 360° C., so that a plurality of nanoparticles comprising zirconium and zinc may be formed that are precursors to the plurality of un-recrystallized regions formed after the deformation process. In certain variations, the methods of the present disclosure may include further hot processing to facilitate formation of nano-scale zinc-zirconium (ZnZr) particles that impede recrystallization of zirconium-rich cores, while the remainder of the magnesium-alloy matrix is dynamically recrystallized. A nanoparticle may have a particle size of less than or equal to about 1 micrometer (μm), optionally less than or equal to about 750 nm, optionally less than or equal to about 500 nm, optionally less than or equal to about 250 nm, optionally less than or equal to about 150 nm, optionally less than or equal to about 100 nm, and in certain variations, optionally less than or equal to about 50 nm.

A suitable high temperature/hot process may be conducted at a temperature of greater than or equal to about 360° C. As discussed above, the high temperature processing may be conducted as an independent heat treatment or in conjunction with subsequent deformation processes conducted at high temperatures.

In certain aspects, such a hot process may be a hot plastic deformation process having a strain rate may be greater than or equal to about 0.001/s to less than or equal to about 1/s and a temperature may be greater than or equal to about 360° C. to less than or equal to about 420° C. The deformation strain may be greater than or equal to about 50% to less than or equal to about 1,000%. A suitable hot plastic deformation process selected from the group consisting of: extruding, forging (in a cavity of a pair of dies), flow forming, and combinations thereof.

In one variation, the deforming further comprises extruding the preform, forging the preform, and flow forming the preform to form the hot-formed solid magnesium-based alloy component. Such a manufacturing process that includes casting, extruding, forging, and flow forming may be used to form an automotive component, such as a wheel 150 shown in FIG. 3 . FIGS. 4A-4C show a hot-formed magnesium-based alloy component having the above-described bimodal microstructure with a plurality of un-recrystallized domains (designated by arrows) formed from solid precursor particles comprising zirconium/nano-scale zinc-zirconium (ZnZr) particles distributed in a matrix comprising dynamically recrystallized grains. FIG. 4A shows an optical microscopy (OM) scan, FIG. 4B scanning electron microscopy scan (SEM), and FIG. 4C shows an electron backscatter diffraction (EBSD) scan of a bimodal microstructure formed in accordance with certain aspects of the present disclosure.

The present disclosure also contemplates a hot-formed solid magnesium-based alloy component. By hot-formed, it is meant that the component has been subjected to a heat treatment or plastic deformation process at the temperatures as described above, for example, greater than or equal to about 360° C. During the process of making such components, a first type of pre-solidified zirconium-containing particle is formed in the molten alloy in situ during casting as described above. These particles comprising zirconium are then dissolved into the remaining magnesium-alloy matrix. After heat treating following casting or in a preheating process conducted prior to hot forming, nano-scale zirconium-containing particles (e.g., zirconium-zinc particles) are then formed in a magnesium core region. After subjecting the magnesium-alloy material to a deformation process, dynamic recrystallization then occurs to generate a bimodal microstructure having a plurality of un-recrystallized domains formed from the zirconium-rich domains distributed in a matrix comprising dynamically recrystallized grains. As discussed above, the dynamically recrystallized grains may have an average size of greater than or equal to about 0.5 μm to less than or equal to about 10 μm, while the un-recrystallized regions have an average equivalent diameter of greater than or equal to about 10 μm to less than or equal to about 100 μm.

Thus, the hot-formed magnesium-based alloy is subjected to a deformation process at relatively high temperatures to form coarse un-recystallized (e.g., undynamically recrystallized (un-DRXed)) domains embedded in refined dynamically recrystallized (DRX) domains of the magnesium-based alloy. The coarse un-recystallized domains in the magnesium alloy have a strong texture strengthening effect. In addition, the nano-scale zirconium-containing (Zr—Zn) particles that impede recrystallization have a precipitation strengthening effect. In this manner, a bimodal microstructure including both dynamically recrystallized regions and un-recrystallized regions is formed that improves strength and ductility of the magnesium-based alloy.

The magnesium-based alloy comprises zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 4 wt. % of the magnesium-based alloy, zirconium (Zr) at greater than or equal to about 0.62 wt. % to less than or equal to about 1 wt. % of the magnesium-based alloy, total impurities at less than or equal to about 0.1 wt. % of the magnesium-based alloy, and a balance of magnesium (Mg).

In certain aspects, the magnesium-based alloy comprises zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 3.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy.

In certain aspects, the magnesium-based alloy comprises zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 2.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy.

In some example embodiments, the magnesium alloy consists essentially of zinc (Zn), zirconium (Zr), optional impurities/contaminants, and magnesium (Mg) at any of the levels specified above.

In some example embodiments, the magnesium alloy consists of zinc (Zn), zirconium (Zr), optional impurities/contaminants, and magnesium (Mg) at any of the levels specified above.

In other aspects, the plurality of solid nanoparticles comprising zirconium, for example, comprising zirconium and zinc, is evenly or homogeneously distributed in the matrix. The plurality of un-recrystallized domains may occupy greater than or equal to about 5% by area to less than or equal to about 50% by area of the hot-formed solid magnesium-based alloy component. Each nanoscale ZnZr particle may have an average size of greater than or equal to about 1 nm to less than or equal to about 1 μm, while the plurality of un-recrystallized domains have an average equivalent diameter of greater than or equal to about 10 micrometer (μm) to less than or equal to about 100 μm.

In certain variations, at least one region of the hot-formed solid magnesium-based alloy component having the bimodal microstructure has a yield strength of greater than or equal to about 170 MPa, optionally greater than or equal to about 175 MPa, optionally greater than or equal to about 180 MPa, and in certain variations, optionally greater than or equal to about 185 MPa.

In certain variations, at least one region of the hot-formed solid magnesium-based alloy component having the bimodal microstructure has an elongation at break of greater than or equal to about 15%, optionally greater than or equal to about 16%, optionally greater than or equal to about 17%, optionally greater than or equal to about 18%, optionally greater than or equal to about 19%, and in certain variations, optionally greater than or equal to about 20%.

In one variation, at least one region of the hot-formed solid magnesium-based alloy component has a yield strength of greater than or equal to about 185 MPa and has an elongation of greater than or equal to about 20%.

In certain aspects, the magnesium-based alloy component is an automotive component selected from the group consisting of: an internal combustion engine component, a valve, a piston, a turbocharger component, a rim, a wheel, a ring and combinations thereof.

A comparison of mechanical properties of a comparative magnesium-based alloy with one example of an inventive magnesium-based alloy is discussed herein. The comparative magnesium-based alloy has a zirconium content at 0.5 wt. % and zinc (Zn) at 3 wt. % with a balance of magnesium (Mg) and impurities. The example according to one embodiment in accordance with the present teachings has, zirconium (Zr) at 0.65 wt. %, zinc (Zn) at 3 wt. %, and a balance of magnesium (Mg) and impurities.

FIG. 5A shows the comparative magnesium-based alloy after casting 200 showing a detailed view of magnesium grains 202 on the left and an optical microscopy scan on the right. FIG. 5B shows the inventive magnesium-based alloy also after casting 200 having magnesium grains 202, but further having pre-solidified zirconium-containing precursor particles 204 acting as nucleation site for magnesium grains 202 in the detailed view on the left. The pre-solidified zirconium-containing precursor particles 204 serve as a nucleation site for the magnesium grains 202 in the inventive example, which enables formation of the bimodal microstructure in the final product. These pre-solidified zirconium-containing precursor particles may be partially or fully dissolved in magnesium grains. As shown in the optical microscopy scan on the right, the magnesium grains 202 having an interior with black contrast (designated by the arrows) show nucleated magnesium grains 202 formed on the pre-solidified zirconium-containing precursor particles 204. The pre-solidified zirconium-containing precursor particles 204 are thus dissolved in the subsequent solidification and thus, zirconium is supersaturated and segregated in the core region of grains formed to form zirconium-rich domains.

FIG. 6A shows the comparative magnesium-based alloy after hot deformation 210 showing the beginning optical microscopy scan of the cast preform on the left and an optical microscopy scan after hot deformation on the right. When the as-cast microstructure is further processed, for example, via heat treatment or hot forming processes, including extrusion and forging, zirconium atoms may bond with zinc atoms in the surrounding magnesium matrix to form nano-scale zirconium-containing particles, for example, zirconium-zinc nanoparticles. The resulting nanoscale zirconium-zinc particles then impede a dynamic recrystallization process during hot plastic deformation that then facilitates formation of a bimodal microstructure in the final product. The magnesium-based alloy may undergo dynamic recrystallization during the hot deformation process.

In FIG. 6A, the entire microstructure is dynamically recrystallized. FIG. 6B shows the inventive magnesium-based alloy also after hot deformation 210 with an initial optical microscopy scan of the cast preform on the left and an optical microscopy scan after hot deformation on the right.

In FIG. 6B, the zirconium-rich domains within the magnesium grains can form nano-scale zinc-zirconium particles that impede recrystallization. In this manner, the zirconium-rich domains become un-recrystallized regions after deformation, as shown by the arrows 212 that are distributed within a matrix 214 of dynamically recrystallized magnesium grains. The bimodal microstructure thus formed has relatively high yield strength and relatively high elongation/ductility.

In this manner, a bimodal microstructure is formed, where the un-recrystallized regions formed where zirconium-rich domains were present after casting, thus serving to enhance strength and ductility/elongation of the hot-formed magnesium-based alloy solid component. For example, a yield strength of the comparative magnesium-based alloy with a conventional microstructure like that shown in FIG. 6A is about 155 MPa and has an elongation of about 13%. However, a yield strength of the inventive magnesium-based alloy with a bimodal microstructure like that shown in FIG. 6B is about 187 MPa and has an elongation of about 20%.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. 

What is claimed is:
 1. A method of making a magnesium-based alloy component comprising: casting a magnesium-based alloy by melting a magnesium-based alloy in a furnace having a temperature (T) of greater than or equal to T=650° C.+(500×((C_(Zr)−0.6))° C., where C_(Zr) represents a concentration of zirconium (Zr) at greater than or equal to about 0.62 wt. % to less than or equal to about 1 wt. % of the magnesium-based alloy, and the magnesium-based alloy further comprises zinc (Zn) at greater than or equal to about 2 to less than or equal to about 4 wt. % of the magnesium-based alloy, total impurities at less than or equal to about 0.1 wt. % of the magnesium-based alloy, and a balance of magnesium (Mg); solidifying the magnesium-based alloy into a preform comprising a plurality of zirconium-rich domains distributed in grains of a magnesium-alloy matrix; and subjecting the preform to a temperature of greater than or equal to about 360° C. and a deformation process that facilitates selective dynamic recrystallization to create a bimodal microstructure in the magnesium-based alloy component to form a plurality of un-recrystallized regions distributed in a matrix comprising dynamically recrystallized grains having an average size of greater than or equal to about 0.5 micrometers to less than or equal to about 10 micrometers.
 2. The method of claim 1, wherein after the subjecting the preform to the temperature of greater than or equal to about 360° C., a plurality of nanoparticles comprising zirconium and zinc are formed that are precursors to the plurality of un-recrystallized regions formed after the deformation process.
 3. The method of claim 1, wherein the subjecting the preform to the temperature of greater than or equal to about 360° C. and the deformation process occur concurrently.
 4. The method of claim 1, wherein the casting is conducted at a temperature (T) of greater than or equal to about 700° C. to minimize formation and settling of a plurality of solid particles comprising zirconium in the molten magnesium-based alloy.
 5. The method of claim 1, wherein the deformation process is selected from the group consisting of: extruding, forging, flow forming, and combinations thereof.
 6. The method of claim 1, wherein the magnesium-based alloy comprises zinc (Zn) at greater than or equal to about 2 to less than or equal to about 3.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy.
 7. The method of claim 1, wherein the magnesium-based alloy comprises zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 2.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy.
 8. The method of claim 1, wherein the magnesium-based alloy component has the plurality of un-recrystallized regions homogeneously distributed in the matrix.
 9. The method of claim 1, wherein the plurality of un-recrystallized regions occupy greater than or equal to about 15% by area to less than or equal to about 40% by area of the magnesium-based alloy component and the plurality of un-recrystallized regions has an average equivalent diameter of greater than or equal to about 10 micrometers to less than or equal to about 100 micrometers.
 10. The method of claim 1, wherein at least one region of the magnesium-based alloy component has a yield strength of greater than or equal to about 170 MPa and an elongation of greater than or equal to about 15%.
 11. The method of claim 1, wherein at least one region of the magnesium-based alloy component has a yield strength of greater than or equal to about 185 MPa and has an elongation of greater than or equal to about 20%.
 12. The method of claim 1, wherein the magnesium-based alloy component is an automotive component.
 13. A hot-formed solid magnesium-based alloy component comprising: a bimodal microstructure having a plurality of un-recrystallized domains distributed in a matrix comprising dynamically recrystallized grains having an average size of greater than or equal to about 0.5 micrometers to less than or equal to about 10 micrometers, wherein a magnesium-based alloy comprises zinc (Zn) at greater than or equal to about 2 to less than or equal to about 4 wt. % of the magnesium-based alloy, zirconium (Zr) at greater than or equal to about 0.62 wt. % to less than or equal to about 1 wt. % of the magnesium-based alloy, total impurities at less than or equal to about 0.1 wt. % of the magnesium-based alloy, and a balance of magnesium (Mg).
 14. The hot-formed solid magnesium-based alloy component of claim 13, wherein the magnesium-based alloy comprises zinc (Zn) at greater than or equal to 2 to less than or equal to about 3.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy.
 15. The hot-formed solid magnesium-based alloy component of claim 13, wherein the magnesium-based alloy comprises zinc (Zn) at greater than or equal to about 2 wt. % to less than or equal to about 2.5 wt. % of the magnesium-based alloy and zirconium (Zr) at greater than or equal to about 0.65 wt. % to less than or equal to about 0.8 wt. % of the magnesium-based alloy.
 16. The hot-formed solid magnesium-based alloy component of claim 13, wherein the plurality of un-recrystallized domains is homogeneously distributed in the matrix, wherein the plurality of the plurality of un-recrystallized domains occupy greater than or equal to about 15% by area to less than or equal to about 40% by area of the hot-formed solid magnesium-based alloy component, and the plurality of un-recrystallized domains has an average equivalent diameter of greater than or equal to about 10 micrometers to less than or equal to about 100 micrometers.
 17. The hot-formed solid magnesium-based alloy component of claim 13, wherein at least one region of the hot-formed solid magnesium-based alloy component has a yield strength of greater than or equal to about 170 MPa and an elongation of greater than or equal to about 15%.
 18. The hot-formed solid magnesium-based alloy component of claim 13, wherein at least one region of the hot-formed solid magnesium-based alloy component has a yield strength of greater than or equal to about 185 MPa and has an elongation of greater than or equal to about 20%.
 19. The hot-formed solid magnesium-based alloy component of claim 13, wherein the hot-formed solid magnesium-based alloy component is an automotive component.
 20. The hot-formed solid magnesium-based alloy component of claim 13, wherein the hot-formed solid magnesium-based alloy component is a wheel. 